In a so-called a lithography process which transfers a pattern formed on a mask or a reticle (hereinafter, collectively referred to as the “mask”) to a photosensitive substrate, an exposure apparatus is used which projects the pattern of the mask held to a mask stage to each shot on the photosensitive substrate via a projection optical system.
In recent years, in the exposure apparatus, higher resolution of the projection optical system is required to correspond to higher integration of the pattern. The resolution of the projection optical system is increased as the used exposure wavelength is shortened and a numerical aperture of the projection optical system is increased. Accordingly, the exposure wavelength used in the exposure apparatus is shortened year by year, and the numerical aperture of the projection optical system is also increased. Currently, a mainstream exposure wavelength is 248 nm of KrF excimer laser and 193 nm of ArF excimer laser. Moreover, when exposure is performed, similar to the resolution, a depth of focus (DOF) is also important. The resolution R and the depth of focus δ are represented by the following Expressions (1) and (2).R=k1·λ/NA  (1)δ=±k2·λ/NA2  (2)
Here, λ is the exposure wavelength, NA is the numerical aperture of a projection exposure system, and k1 and k2 are process coefficients.
According to Expressions (1) and (2), in order to increase the resolution R, it is understood that the depth of focus δ is decreased if the exposure wavelength λ is decreased and the numerical aperture NA is increased.
If the depth of focus δ is narrowed too much, it is difficult to make a substrate surface coincide with an image surface of the projection optical system, and there is a concern that a focus margin may be insufficient when the exposure operation is performed. Accordingly, as a method which substantially shortens the exposure wavelength and widens the depth of focus, for example, a liquid immersion method disclosed in Patent Document 1 below is suggested. In the liquid immersion method, liquid such as pure water or organic solvent is filled between a mask of a projection optical system and a substrate surface, resolution is improved using a wavelength of exposure light in the liquid becoming 1/n (n is a refractive index in air and approximately 1.2 to 1.6 in general) in air, the depth of focus is enlarged to be approximately n times, and a liquid immersion exposure apparatus which exposes a substrate in high resolution by the liquid immersion method is used.